Make tens first

a + b + c + d

The trick, step by step

1. Look for pairs that make round numbers: 17 + 13 = 30.

   171330

2. And 26 + 4 = 30.

   26430

3. Add the round numbers: 30 + 30 = 60.

   60

given working answer taken away

Why it works

You can add numbers in any order and group them any way you like (addition is commutative and associative), so pair up the ones that make tens or hundreds first.